অ is the first vowel in the Bengali alphabet, and by extension related alphabets such as Assamese. This letter is used to represent the open-mid back rounded vowel, and while equivalent to the inherent 'a' found in many other Indian languages (as found aside consonants which do not otherwise have a marked vowel), has this phonemic difference in common with many Eastern Indo-Aryan languages such as Assamese and Oriya.
Character representation
By virtue of being equivalent to the inherent 'a', this is the only vowel without a combining form and thus has only one codepoint.
References
A (Bengali) Wikipedia(Text) CC BY-SA